Field of the Invention
The present invention relates to an internal EGR amount calculation device for an internal combustion engine, which calculates an internal EGR amount of the engine.
Description of the Related Art
Conventionally, as an internal EGR amount calculation device for an internal combustion engine, one described in Japanese Laid-Open Patent Publication (Kokai) No. 2004-251182 is known. This internal EGR amount calculation device calculates an internal EGR amount by adding a blow back gas amount to a residual burned gas amount. This residual burned gas amount is an amount of burned gas remaining in a cylinder and is calculated specifically using the equation of state of gas.
Further, the blow back gas amount represents an amount of burned gas which has once flowed into an intake passage from an exhaust passage and then has been blown back into the cylinder, due to the pressure difference between the intake passage and the exhaust passage. This blow back gas amount is calculated using the nozzle equation, by regarding a flow passage through which the burned gas flows, as a nozzle.
The nozzle equation includes an integrated value of an effective opening area, and the integrated value of the effective opening area is calculated as a function of the length of a valve overlap period (i.e. a crank angle from a valve opening timing of an exhaust valve to a valve closing timing of an intake valve) and an engine speed.
In general, in the case of an internal combustion engine in which the valve overlap period is changed, although the blow back gas amount has a high correlation with the length of the valve overlap period, the blow back gas amount has characteristics that it changes due to a timing position relationship between the valve overlap period and the exhaust top dead center, even if the valve overlap period is the same. For example, assuming that the valve overlap period is the same, when a comparison is made between a state in which the proportion of a time period after the exhaust top dead center is smaller and a state in which the proportion of a time period after the exhaust top dead center is larger, the blow back gas amount is larger in the state in which the proportion of the period after the exhaust top dead center is larger, due to the fact that the burned gas is directly returned to the cylinder from the exhaust passage (see FIGS. 8 and 10, referred to hereinafter).
In contrast, in the case of the internal EGR amount calculation device described in above Japanese Laid-Open Patent Publication (Kokai) No. 2004-251182, the equation for calculating the blow back gas amount only uses the function of the length of the valve overlap period and the engine speed, and the above-mentioned characteristics that the blow back gas amount changes due to the timing position relationship between the valve overlap period and the exhaust top dead center is not taken into consideration. Therefore, when the blow back gas amount changes as mentioned above, a calculation error of the blow back gas amount increases, so that the calculation accuracy of the internal EGR amount decreases.